1. Field of the Invention
The present invention relates to the field of flat panel displays. More specifically, the present invention relates to a flat panel display and methods for forming a flat panel display having a frit seal that is protected from reaction with a passivation layer.
2. Related Art
A Cathode Ray Tube (CRT) display generally provides the best brightness, highest contrast, best color quality and largest viewing angle of prior art displays. CRT displays typically use a layer of phosphor that is deposited on a thin glass faceplate. These CRTs generate a picture by using one to three electron beams that generate electrons that are scanned across the phosphor in a raster pattern. The phosphor converts the electron energy into visible light so as to form the desired picture. However, prior art CRT displays are large and bulky due to the large vacuum tubes that enclose the cathode and extend from the cathode to the faceplate of the display. Therefore, other types of display technologies such as active matrix liquid crystal display, plasma display and electroluminescent display technologies have been used in the past to form thin displays.
Recently, a thin flat panel display has been developed that uses the same process for generating pictures as is used in CRT devices. These thin flat panel displays use a backplate including a matrix structure of rows and columns of electrodes. One such flat panel display is described in U.S. Pat. No. 5,541,473 titled GRID ADDRESSED FIELD EMISSION CATHODE that is incorporated herein by reference as background material. Typically, the backplate is formed by depositing a cathode structure (electron emitting) on a glass plate. The cathode structure includes emitters that generate electrons. The backplate typically has an active area within which the cathode structure is deposited. Typically, the active area does not cover the entire surface of the glass plate, leaving a thin strip that extends around the glass plate. Electrically conductive traces extend through the thin strip to allow for connectivity to the active area.
Prior Art FIG. 1 illustrates a flat panel display device 100. A backplate 105 is shown with an active area 110. Glass frit bars for sealing the backplate 105 to a faceplate (not shown) are deposited within the thin strip area 160 that does not contain the active area 110. This thin strip area is also called the encapsulant region 160. The glass frit bars can be partitioned into glass frit bars, 120, 130, 140, and 150.
Additionally, electrically conductive traces (not shown) extend through the thin strip area 160 to allow for connectivity to row and column electrodes in the active area 110. A passivation layer, for example, composed of silicon nitride (SixNy) can be deposited over the electrically conductive traces for protecting the electrodes from damage and contamination during the sealing process.
Prior art flat panel displays include a thin glass faceplate having one or more layers of phosphor deposited over the interior surface thereof. The faceplate is typically separated from the backplate by about 0.1 to 5 millimeters. The faceplate includes an active area within which the layer (or layers) of phosphor is deposited. A thin strip that does not contain phosphor extends from the active area to the edges of the glass plate. The faceplate is attached to the backplate using a glass seal.
In one prior art process, glass frit bars (e.g., frit bars 120, 130, 140, and 150), or bars with a thin layer of frit material, are placed within the thin strip in a frame-shape such that the glass frit bars surround the active area of the faceplate. The backplate is then placed over the faceplate. The flat panel display assembly is then aligned and may be tacked so as to hold the faceplate and the backplate in their proper alignment. Typically, four tacks are used: one in each corner of the flat panel display assembly, for example. The thickness of the frit bars is less than the distance between the faceplate and the backplate such that there is a gap between the top of the glass frit and the bottom of the faceplate. This gap is typically about one to two thousandths of an inch.
The assembly is then placed in an oven and heated to the bias temperature of the glass frit bars (this is done to minimize stress fracturing resulting from the sudden increase in temperature). A laser is then used to melt the glass frit bars. The heat of the laser melts the glass frit locally and causes the glass frit to expand such that the glass frit contacts the backplate, thereby wetting the surface of the backplate and forming a xe2x80x9cbead.xe2x80x9d The laser is moved, drawing the bead around the surface of the glass frit until the desired seal is formed.
Also, an oven sealing process can be used rather than a laser for melting the glass frit and forming the desired seal between the backplate and the faceplate.
The melting of the glass frit forms an enclosure that is subsequently evacuated so as to produce a vacuum between the active area of the backplate and the active area of the faceplate. In operation, individual regions of the cathode are selectively activated to generate electrons which strike the phosphor so as to generate a display within the active area of the faceplate. These flat panel displays have all of the advantages of conventional CRT displays but are much thinner.
Prior art flat panel display fabrication processes often result in a defective seal between the faceplate and the backplate, such as the backplate shown in Prior Art FIG. 1B. Defective seals result from outgas species that condense on the metal electrodes. This condensation creates an unwettable surface when sealing the frontplate to the backplate with the glass frit bars. As such, leakage of the vacuumed enclosure between the faceplate and the backplate can occur rendering the display device unusable or defective. In other cases, the frit material can dissolve the row and column electrodes.
In particular, Prior Art FIG. 1B illustrates a side sectional view of the backplate 105 taken along a line Xxe2x80x94X in FIG. 1A. As shown, nitrogen outgas species creates defections within the seal attaching faceplate 115 to backplate 105 that lead to porous leak paths 170 through the seal attaching the backplate 105 to the faceplate 115 of the flat panel display.
The nitrogen outgas species is a product of the spontaneous reaction between the frit bar 130 and the silicon nitride layer 180 along the line Xxe2x80x94X as shown in Prior Art FIG. 1B. As discussed previously, the silicon nitride layer 180 is a passivation layer that protects electrodes and or their corresponding electrically conductive traces leading into the active area 110.
During the sealing process for attaching the faceplate 115 to the backplate 105, the reaction between the frit bar 130 and the silicon nitride layer 180 is most pronounced. Contained within the silicon nitride layer 180 is lead oxide. The lead oxide spontaneously reacts with silicon nitride (SixNy) in the silicon nitride layer 180. The reaction as shown in equation (1) below, has a negative free energy value indicating the reaction is spontaneous at temperatures used for sealing the faceplate 115 to the backplate 105 of a field emission display device. As a result, nitrogen outgas species is readily produced leading to porous leak paths 170 and a degradation in the seal between the faceplate 115 and the backplate 105, particularly in the seal between the frit bar 130 and the silicon nitride layer 180.
xe2x80x83Si3N4+6PbO⇄3SiO2+6Pb+2N2xe2x80x83xe2x80x83(1)
Most particularly, during an oven sealing process, the reaction as shown above in Equation (1) occurs over a greater period of time in relation to the laser sealing process. As such, more nitrogen outgas species is produced leading to more porous leaks 170 and greater degradation of the sealing between the faceplate and the backplate.
Additionally, the nitrogen outgas species bubbles to the surface of the silicon nitride layer 180 that is adjacent to the localized frit bar 130 as shown in Prior Art FIG. 1B. The bubbling of the nitrogen outgas species in the interface between the frit bar 130 and the silicon nitride layer 180 indicates poor wettability between the frit bar 130 and the silicon nitride layer 180. As such, degradation of the seal between the frit bar 130 and the silicon nitride layer 180 occurs.
Moreover, nitrogen gas is produced when depositing the silicon nitride passivation layer 180 over the electrically conductive traces leading to the row and column electrodes in the active region. The silicon nitride passivation layer 180 is deposited by a plasma enhanced chemical vaporization process (PE CVD). In the PE CVD process, hydrogen is produced which leads to production of nitrogen gas from the silicon nitride that is deposited. This nitrogen gas bubbles to the surface of the silicon nitride passivation layer and creates porous leaks (e.g., porous leaks 170) in the interface between the silicon nitride passivation layer 180 and the localized glass frit bar 130.
Thus, a need exists for a sealing frame process that results in lower leak rates. Another need exists for a sealing process that creates a more reliable seal between the backplate and the faceplate. Still another need exists for a sealing process that provides better wettability for the glass frit bar in sealing a faceplate to a backplate of a field emission display device.
The present invention provides a method for protecting the glass frit bar from reacting with a silicon nitride passivation layer when sealing a faceplate to a backplate on a field emission display device. Also, the present invention provides a method that achieves the above accomplishment and which also provides for a sealing frame process that results in lower leak rates. Additionally, the present invention provides a method that achieves the above accomplishments and which also provides for a sealing frame process that creates a more reliable seal between the faceplate and a backplate. Moreover, the present invention provides a method that achieves the above accomplishments and which also provides for better wettability for the glass frit bar in sealing the faceplate to a backplate of a field emission display device.
Specifically, one embodiment of the present invention discloses a method for attaching a faceplate and a backplate of a field emission display device. Specifically, a silicon nitride passivation layer is prevented from reacting with a glass frit sealing material during an oven sealing or laser sealing process. The silicon nitride passivation layer protects row and column electrodes in the display device. A barrier material fully encapsulates the silicon nitride passivation layer to prevent reaction with lead oxide present in the glass frit sealing material. In one embodiment, silicon dioxide is the barrier material. In another embodiment, spin-on-glass is the barrier material. In still another embodiment, cermet is the barrier material.
In one embodiment of the present invention, the method includes creating a cathode backplate structure that includes row and column electrodes. A silicon nitride passivation layer is deposited over electrical traces in the encapsulant region that lead to the row and column electrodes in the active region of the cathode backplate structure. The encapsulant region is the area used for attaching the cathode backplate structure to a faceplate in a field emission display device.
The silicon nitride passivation layer is then encapsulated with a barrier material. In one embodiment, the barrier material is silicon dioxide. In still another embodiment, the barrier material is spin-on-glass. In another embodiment, the barrier material is a cermet mixture of chromium oxide and quartz. In one embodiment the cermet mixture has an approximate composition of sixty-two percent chromium oxide (Cr2O3) and thirty-eight percent quartz (SiO2).
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.